Presidents
Corner

Weather, Climate, and
the Evolving U.S. Climate Change Science Program

"Climate is what you expect; weather is what you get."
Robert Heinlein, Time Enough for Love

Although it sums up how many think of weather and climate, this adage
sets the two apart in a way that is detrimental to the study and prediction
of both. Some people study weather, others study climate.
Priorities are often set for one or the other, rather than looking at
both. In reality, the two are intimately connected; climate on any time
scale is the integral, or summation, of all weather over that time scale.
The weather is the devil in the details of climate.

The connection between weather and climate was one of several recurring
themes in the U.S. Climate Change Science Program workshop held in Washington,
D.C., 35 December 2002. The CCSP (see On the Web)
incorporates the U.S. Global Change Research Program, which was established
in 1989 and authorized by Congress in 1990. It also includes the new
Climate Change Research Initiative, sponsored by the White House, which
seeks to accelerate progress in understanding climate change and to
produce decision relevant information in two to five years.

The CCSP Planning Workshop for Scientists and Stakeholders was organized
and led by James Mahoney, assistant secretary of commerce for oceans
and atmosphere and director of the CCSP. Many senior members of the
administration and international leaders presented keynote speeches.
Twenty-four breakout groups met to discuss various aspects of the draft
of the strategic plan for U.S. climate change and global change research.
Altogether, approximately 1,200 people participatedmore than twice
the number originally expected.

The weather-climate connection was apparent in the discussions and
conclusions coming from the workshop. Many users of climate information,
from California to New Jersey, emphasized their need for detailed scenarios
of potential future climate change on the regional scale. They were
interested in possible changes in precipitation, drought, severe storms,
wildfires, and other high-impact phenomena, with the changes ideally
expressed as probability distribution functions (PDFs).

The interests and requirements of users of weather and climate information
point to a number of high-priority items for the CCSP. For example,
to produce what these customers are asking for will require enormous
increases in computer power, in order to

increase the horizontal and vertical resolution of climate models,

assimilate the enormous volumes of in situ and remotely sensed
observations,

increase the number of physical, biological, and chemical processes
in the models, and

run ensembles of climate projections to generate PDFs of key variables
and to define the uncertainties in the projections.

How much more computer power will we need to accomplish the above?
One estimate is a million or more times beyond that now provided by
the worlds most powerful computer, Japans Earth Simulator,
which delivers about 50 teraflops of useful computer power. (This is
approximately 10 times the power of NCARs present suite of computers,
which places 10th in the world according to rankings at www.top500.org.)

Another important priority emerging from the workshop was the urgent
need for a robust climate observing system. The global system that has
evolved over the previous century for operational weather prediction
has provided an extremely important picture of global climate. However,
observations that meet the needs of weather prediction often have serious
flaws when it comes to defining the climate record.

Radiosondes, one of the mainstays of the global observational network
since the 1930s, are a case in point. Radiosondes provide measurements
with high vertical resolution of temperature and wind throughout the
troposphere and of water vapor in the lower half of the troposphere.
The accuracy of these data is good enough to meet the needs of numerical
weather prediction. However, there are significant shortcomings when
applied to climate. Radiosondes provide uneven coverage globally, since
few are launched over the oceans; they do not resolve the diurnal cycle,
because most stations launch sondes only once or twice daily; and they
do not provide much information from the stratosphere.

Over the years, various temperature and humidity sensors have been
used on radiosondes by different countries and by the same countries
at different times. Each type of radiosonde has unique error characteristics
for temperature and water vapor. The differences, which may be on the
order of a degree Celsius and 1050% in relative humidity, may
be small enough not to affect weather prediction significantly, but
they are on the same order as the magnitude of decadal trends in climate.
Over the past couple of decades, satellites have complemented the radiosonde
observations and have addressed some of the shortcomings above. However,
satellite estimates of temperature have deficiencies of their own (for
example, they generally have poor vertical resolution and suffer from
instrumental drift, changes in calibration and orbit, and similar problems).
The discrepancies between temperature trends derived from radiosondes
versus satellites has been a major source of controversya topic
discussed by one of the breakout groups in the CCSP workshop.

Recent results from the International H2O Project (IHOP2002) experiment
(see figure) showed major limitations of operational radiosondes in
measuring the relative humidity in the upper troposphere. Although this
region is so cold that there is not much water vapor of significance
for weather prediction, even small amounts of water vapor and cirrus
clouds in the upper troposphere are extremely important for climate,
as they strongly affect Earths radiation budget.

Over a dozen radiosonde launches during IHOP2002 bore not one, but
two instrument packages. Along with the standard set of sensors, these
launches also carried a reference radiosonde package designed
at NCARs Atmospheric Technology Division. The reference package
included a state-of-the-art humidity sensor nicknamed Snow White. The
sensor used a chilled-mirror technique to measure humidity far more
accurately than the two sensor types used on the most popular operational
radiosondes.

At heights between about 10 and 14 kilometers (69 miles), Snow
White often found a thin layer of high humidity, indicating the possible
presence of cirrus clouds. In some cases, satellite imagery and/or data
from a NASA airborne lidar have confirmed that cirrus clouds were present.
Neither of the two standard humidity sensors showed the high moisture
content present at these levels, however. Indeed, one of the two sensor
types regularly indicated relative humidity below 30% throughout the
upper atmosphere, even in cases where cirrus clouds were known to be
present.

If this finding turns out to be robust across other regions and circumstances,
it will have profound implications for climate monitoring. By and large,
cirrus clouds act to warm the lower atmosphere. It is possible that
decades of climate records have underestimated the amount of cirrus
clouds in the global atmosphere. Future satellite systems may help,
but the layers of moisture and clouds at these levels are often too
thin for satellites to resolve. The challenge for the weather and climate
communities is to work together to recognize and correct such problems.
If a radiosonde network is designed for use in forecasting, how can
the climate community ensure that its needs are met as well?

At right, a composite of six soundings launched from Dodge City,
Kansas, during IHOP2002 shows major differences between the high-precision
Snow White humidity sensor (red) and the hygristors used in ATD reference
radiosondes (blue) and in the National Weather Service's operational Sippican (VIZ) radiosondes (green). At left, the RS80-H
humicap device in the Vaisala sondes (green) shows a weak reflection
of the high upper-level humidity observed in six soundings launched
from the Homestead site in the eastern Oklahoma Panhandle. (Illustration
courtesy Junhong Wang, NCAR.)

This question is a timely one. The National Weather Service (NWS)
is replacing its present radiosondes with new GPS sondes over the next
four years. While the GPS capability will improve wind measurements,
some of the new sondes will continue to use carbon hygristors rather
than the Vaisala RS80-H humidity sensors that are now carried on about
two-thirds of all U.S. sondes. The IHOP2002 results indicate that carbon
hygristors show no response at all to humidity at temperatures below
30†C (22°F), whereas the RS80-H sensors show at least
some limited response. Although the new radiosondes could in fact be
the best fit for the stated NWS requirements, they fall short of what
the climate research community requires to accurately assess global
change. -Rick Anthes